Method for producing si1-ygey based zones with different contents in ge on a same substrate by condensation of germanium

ABSTRACT

The disclosure relates a method for producing a microelectronic device including a plurality of based Si 1-y Ge y  semi-conductor zones (where 0&lt;y≦1) have different respective Germanium contents, comprising the steps of:
         a) formation on a plurality of Si based semi-conductor zones with different thicknesses resting on a substrate, of a Si 1-y Ge y  based semi-conductor layer (where 0&lt;x&lt;1 and x&lt;y),   b) oxidation of the Si 1-y Ge y  based semi-conductor layer.

TECHNICAL FIELD

The invention relates to a method for producing an improved microelectronic device in which Si_(1-y)Ge_(y) based semi-conductor blocks of different compositions are formed on a same substrate.

The invention may favour the integration, on a same semi-conductor substrate, or a same chip, or a same semi-conductor plate, simultaneously of a first plurality of transistors comprising an active layer based on a material that is optimised for the conduction of electrons and of a second plurality of transistors comprising an active layer based on a material that is optimised for the conduction of holes, wherein the performances of the transistors of the first plurality of transistors and the second plurality of transistors are equalised.

PRIOR ART

In the aim of improving the performances of the transistors, especially in terms of rapidity, there is a known technique of forming for the latter strained channels which have improved mobility for the electrons or holes. A strained channel is generally formed of at least one first semi-conductor layer based on a first semi-conductor material onto which a mechanical tensile or compressive strain is applied via another layer that is underlying or is located on the said first semi-conductor layer and for example based on a crystalline semi-conductor material that has a different mesh parameter from said first semi-conductor material. An appropriate semi-conductor material to favour good mobility of the holes often does not favour the mobility of the electrons. Consequently, a tensile or compressive strain applied onto a given semi-conductor material allows a greater increase in this material of the mobility of one type of carrier with respect to the other. For microelectronic devices such as CMOS technology circuits, including one or several transistors permitting main conduction by holes and one or several other transistors permitting main conduction by electrons, it is difficult to be able to increase, using strain channels, both the mobility of the load carriers for the N type transistors and for the P type transistors. In this type of circuit, it is furthermore generally preferable for the performances of the N type transistors and those of the P type transistors to be equalised in terms of current in the conductive state or in terms of mobility, which creates an additional difficulty. One solution consists of creating a microelectronic device equipped with semi-conductor zones based on different semi-conductor materials on a same substrate, and in particular semi-conductor zones based on a semi-conductor material favouring conduction by electrons and semi-conductor zones based on another semi-conductor material favouring conduction by holes.

The document U.S. Pat. No. 6,600,170 presents a creation on a solid semi-conductor substrate, with a semi-conductor zone with a Si based strained channel for a NMOS transistor and a semi-conductor zone with a Si based Ge or Ge strained channel for a PMOS transistor. Such architecture allows equalised performances to be obtained between the NMOS and PMOS transistors. Especially in terms of consumption, the performances of the transistors in such a structure are not optimal. Short channel effects as well as latching up may occur especially in the transistors.

The document FR 04 50889 presents a microelectronic method for producing active semi-conductor zones of different natures directly on an insulator. This method comprises steps consisting of: forming on a first substrate, of at least one first semi-conductor zone in one first crystalline semi-conductor material, and of at least one second semi-conductor zone in one second crystalline second semi-conductor material that is different from the first crystalline semi-conductor, then covering said first and second zones with an insulating thickness, then assembling a second substrate on said insulating thickness, then eliminating the first substrate. Such a method includes a large number of intermediate steps.

There is also the problem of finding a new microelectronic method for producing on a same substrate a set of semi-conductor zones of different compositions, including for example several semi-conductor zones based on a material favouring the conduction by electrons and several other semi-conductor zones based on a material favouring the conduction by holes.

DESCRIPTION OF THE INVENTION

The invention relates to a method for producing a microelectronic device including a plurality of based Si_(1-y)Ge_(y) semi-conductor zones (where 0<y≦1) with different respective contents in Germanium.

In one aspect, the invention relates to a method comprising steps of:

a) formation on a plurality of Si based semi-conductor zones and with different thicknesses resting on a substrate, of a Si1-xGex based semi-conductor layer (where 0<x<1 and x<y),

-   -   b) oxidation of the Si_(1-y)Ge_(y) based semi-conductor layer.

The Si based semi-conductor zones of different thicknesses may be zones of a Si based layer forming a bevel.

The form of the bevel of the Si based layer may be fabricated prior to step a) by:

formation of a mask in the form of a bevel on the Si based layer,

oxidation of the Si layer through the said mask in the form of a bevel,

removal of said mask in the form of a bevel.

The mask in the form of a bevel may be based on an insulating material, for example Si based O₂.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be more clearly understood after reading the description of examples of embodiments provided purely by way of illustration and in no way restrictively, in reference to the appended drawings in which:

FIGS. 1A-1D, illustrate one example of a microelectronic fabrication method on a substrate, of several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) with different compositions of germanium, using at least one step of condensation of the Germanium carried out on a Si_(1-y)Ge_(y) based layer (where 0<x<y<1) protected by a mask of variable thickness,

FIGS. 2A-2D, illustrate another example of a microelectronic fabrication method on a substrate, of several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) with different compositions of germanium, using at least one step of condensation of the Germanium carried out on Si_(1-y)Ge_(y) based semi-conductor zones (where 0<x<y<1) with identical compositions in germanium and protected by a mask of variable thickness,

FIGS. 3A-3C, illustrate another example of microelectronic fabrication method on a substrate, of several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) with different compositions of germanium, using at least one step of condensation of the Germanium carried out on a Si_(1-y)Ge_(y) based layer (where 0<x<y<1) protected by a mask in the form of a bevel,

FIGS. 4A-4B, illustrate another example of microelectronic fabrication method on a substrate, of several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) with different compositions of germanium, using at least one step of condensation of the Germanium carried out on a Si_(1-y)Ge_(y) based layer (where 0<x<y<1) protected by a mask formed by zones of different natures,

FIGS. 5A-5D, illustrate another example of the microelectronic fabrication method on a substrate, in which several oxidation steps of Si_(1-y)Ge_(y) based semi-conductor zones are carried out, using a different mask each time,

FIGS. 6A-6E, illustrate an example of the microelectronic fabrication method of the invention on a substrate, of several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) with different compositions of germanium, using at least one step of condensation of the Germanium carried out on a Si_(1-y)Ge_(y) based layer (where 0<x<y<1) in the form of a bevel,

Identical, similar or equivalent parts of the different figures bear the same numerical references to make it easier to consult the various figures.

The various parts shown on the figures are not necessarily to a uniform scale, in order to make the figures easier to understand.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

An example of fabrication method of a microelectronic device equipped with several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) with different respective compositions in Germanium formed on a same substrate, will now be provided in relation to FIGS. 1A to 1D. The starting material of this method may be a semi-conductor on insulator type substrate, for example of the SOI type (SOI for silicon on insulator) comprising a substrate layer 100 for example Si based, covered with a layer of dielectric material 101 for example a buried oxide layer that is itself covered with a thin semi-conductor layer 102 for example Si based, and with a thickness of between for example 5 and 100 nanometres or for example of around 20 nanometres. On the thin semi-conductor layer 102 another Si_(1-x)Ge_(x) based semi-conductor layer 104, where 0<x<y≦1, for example where x is around 0.1, with a thickness of between for example 50 and 300 or for example between 50 and 100 nanometres or for example between 40 and 75 nanometres is formed, for example by epitaxy (FIG. 1A).

An oxidation mask is then fabricated on the Si_(1-y)Ge_(y) based semi-conductor layer 104. The mask may be formed of at least one layer 108 which may be based on a dielectric material for example such as SiO₂ or Si₃N₄, or a stack of sub-layers of dielectric material(s), for example of a sub-layer based on a first dielectric material such as Si₃N₄, covered with another sub-layer based on a second dielectric material, for example such as SiO₂, or, in another example, a SiO₂ based sublayer, covered with a Si₃N₄ based sublayer. The masking layer 108 may be fabricated by deposition and may have a thickness for example of around 100 nanometres, or between for example 20 and 500 nanometres (FIG. 1B).

Subsequently, a set of holes 110, 112, 114, 116, is fabricated in the layer 108 of dielectric material or in the stack of layers of dielectric materials. The holes 110, 112, 114, 116, formed respectively have a given depth and respectively create a given pattern. Several of the holes 110, 112, 114, 116, formed in the masking layer 108 may have different depths and may create different patterns. The holes 110, 112, 114, 116, may be fabricated for example via a method comprising at least one photolithography or lithography step using an electron beam, or using a “nano-imprint” type method in which the holes 110, 112, 114, 116, are fabricated by stamping, using a stamp equipped with protruding patterns, and that is applied by pressing it on the layer 108 of dielectric material or on the stack of layers of dielectric materials. Certain holes 110, 112, 116, may have a bottom situated at the level of the masking layer 108 and reveal a portion of the latter. The masking layer 108 may also comprise one or several other holes 114 revealing the semi-conductor layer 104. The masking layer 108 equipped with holes 110, 112, 114, 116, of different depths forms a plurality of masking elements or masking blocks of different thicknesses resting on the semi-conductor layer 104 (FIG. 1C).

Subsequently, a localised condensation step of the Ge of the Si_(1-x)Ge_(x) based semi-conductor layer 104 is carried out, for example using an oxidation method such as that described in the document: “A novel fabrication technique of ultrathin and relaxed SiGe buffer layers with high Ge fraction for sub-100 nm strained silicon on insulator MOSFETs, Tezuka and al., Japanese Journal of Applied Physics, vol. 40. p 2866-2874, 2001” or in the document “Selectively formed high mobility SiGe on Insulator pMOSFETs with Ge-rich strained surface channels using local condensation technique, Tezuka and al., Symposium on VLSI Technology, 2004”. To carry out this condensation of the Ge, at least one heat treatment oxidation step of the Si_(1-x)Ge_(x) based semi-conductor layer 104 is carried out at high temperature, and through the mask formed by masking blocks of different thicknesses. The oxidation temperature is preferably lower than the melting temperature of the Si_(1-x)Ge_(x) material of the semi-conductor layer 104. Oxidation in one or several steps and at decreasing temperatures may be carried out.

During the oxidation step, the silicon of the thin semi-conductor layer 102, and the Si_(1-x)Ge_(x) based layer 104, tend to be consumed, forming SiO₂. As the germanium is not or only slightly soluble in the SiO₂, it is rejected at the interface between the thin semi-conductor layer 102 and the buried oxide layer 101.

The masking elements or masking blocks of the layer 108, cause a delay in the oxidation of the semi-conductor layers 102 and 104, that is more or less long depending on their thicknesses respective. The oxidation of a first region 121 of the semi-conductor layer 104, located under a first masking block of a given first thickness e₁, is designed to be slower than that of a second region 122 of the semi-conductor layer 104, located under a second block of the layer 108 of a second thickness e₂ such that e₂≦e₁, which itself is designed to be slower than that of a third region 123 of the semi-conductor layer 104, located under a third block of the layer 108 of a third thickness e₃ such that e₃≦e₂, itself designed to be slower than a fourth region 124 of the layers 104, 108, located in the extension of a hole 114 revealing the semi-conductor layer 104.

Once the oxidation step is complete, several Si_(1-y)Ge_(y) based (where y>x) zones 131, 132, 133, 134 are obtained with respective compositions in Germanium or different respective Germanium contents. The duration of the oxidation step may need to be adapted to suit the concentration in Germanium desired in the different Si_(1-y)Ge_(y) based semi-conductor zones. The first Si_(1-y)Ge_(y) based zone 131, has a first composition in Germanium such that y=y₁, y₁ is for example around 0.1, while the second Si_(1-y)Ge_(y) based zone, has a second composition in Germanium that is different from the first composition and such that y=y₂ where y₂>y₁, y₂ is for example around 0.4, the third Si_(1-y)Ge_(y) based zone has a third composition in Germanium, that is different from the first and the second compositions, and such that y=y₃ where y₃>y₂, y₂ is for example around 0.7, the fourth Si_(1-y)Ge_(y) based zone has a fourth composition in Germanium, that is different from the first, the second and the third composition such that y=y₄ where y₄>y₃, y₄ is for example around 1. As the Si_(1-y)Ge_(y) based zones 131, 132, 133, 134 have different Germanium contents; they may be for example designed to serve respectively as a transistor channel zone. Once the oxidation step is complete, the Si_(1-y)Ge_(y) based zones 131, 132, 133, 134 may be respectively covered by a SiO₂ based layer 135 formed during the oxidation. This layer 135 as well as the masking layer 108 may then be removed (FIG. 1D).

In one possible embodiment (not shown) of the method that we have just described, prior to the depositing of the masking layer 108, the Si_(1-x)Ge_(x) based semi-conductor layer 104 may be covered by a thin Si based semi-conductor layer, for example with a thickness of between 1 and 5 nanometres. This thin semi-conductor layer may prevent the Germanium from being consumed during the oxidation step of the semi-conductor layer 104.

In the example of the method previously described, the material used to form the oxidation mask is a material that is capable of delaying the oxidation of the semi-conductor layer 104, and is not necessarily restricted to a dielectric material.

In one variant of the example of the method previously described, instead of forming a Si_(1-x)Ge_(x) based semi-conductor layer 104, several semi-conductor distinct or disjointed Si_(1-x)Ge_(x) based zones with identical compositions in Germanium are formed on the thin semi-conductor layer 102. Firstly, a masking layer 203 for example based on a dielectric material such as Si₃N₄ is deposited on the thin semi-conductor layer 102. The masking layer 203 may have a thickness for example of around 100 nanometres, or of between for example 50 and 400 nanometres.

In the masking layer 203, a set of holes 210, 212, 214, 216 is then fabricated, revealing the thin semi-conductor layer 102 (FIG. 2A). In one embodiment, the holes 210, 212, 214, 216, may respectively be in the form of a transistor gate or transistor channel.

Subsequently, a set of semi-conductor Si_(1-x)Ge_(x) based zones 204 a, 204 b, 204 c, 204 d, (where 0<x≦1, for example with x of around 0.1) is formed, for example by epitaxy at the bottom of the holes 210, 212, 214, 216, on the thin semi-conductor layer 102 (FIG. 2B).

Then in several holes 210, 212, 214, of the said set of holes, on several zones 204 a, 204 b, 204 c, of the said set of Si based semi-conductor zones Ge, a masking block is formed. A set of several masking blocks 208 a, 208 b, 208 c, respectively filling one hole and covering one Si_(1-x)Ge_(x) based semi-conductor zone is thus formed. The masking blocks 208 a, 208 b, 208 c, may be for example based on a dielectric material such as for example SiO₂. Several masking blocks 208 a, 208 b, 208 c, may have different thicknesses. The masking blocks 208 a, 208 b, 208 c, may have a thickness of between for example 10 nm and 300 nm (FIG. 2C).

An oxidation step is carried out on the SiGe based zones 204 a, 204 b, 204 c, 204 d, at high temperature, through the mask formed by the perforated layer 203 and the masking blocks 208 a, 208 b, 208 c, located in the holes. The oxidation step consumes the silicon of the thin semi-conductor layer 102, and the Si_(1-y)Ge_(y) based zones 204 a, 204 b, 204 c, 204 d. The masking blocks 208 a, 208 b, 208 c, depending on their respective thicknesses, are likely to delay the oxidation or prevent the oxidation of the semi-conductor layer 102 and the zones 204 a, 204 b, 204 c, 204 d.

Once the oxidation step is complete, several Si_(1-y)Ge_(y) based zones 231, 232, 233, 234, are obtained with different respective compositions in Germanium, for example such that 0.1≦y≦1. The Si_(1-y)Ge_(y) based zones 231, 232, 233, 234 may be respectively covered by a SiO₂ based layer formed during oxidation.

The masking blocks 208 a, 208 b, 208 c, and the SiO₂ based layer formed during oxidation may then be removed. In the case where the layer 203 is to be kept, the removal may be carried out by selective etching of the layer 203. If for example the masking blocks 208 a, 208 b, 208 c are SiO₂ based and the layer 203 is Si₃N₄ based, the selective etching may be carried out wet using for example HF, or for example by plasma etching with C₄F₈ and a mixture of argon and oxygen (FIG. 2D).

In one possibility, the Si_(1-y)Ge_(y) based zones 231, 232, 233, 234, with different respective compositions in Germanium may be respectively designed to form a transistor channel. Transistors equipped with Si_(1-y)Ge_(y) based channel zones (where y>x) and different respective compositions in Germanium may thus be fabricated. To complete the formation of transistors, a gate may be made on each Si_(1-y)Ge_(y) based zone 231, 232, 233, 234. To this end, following the removal of the masking blocks 208 a, 208 b, 208 c, and the layer 235, a dielectric gate material, for example HfO₂, may be first deposited in the holes 210, 212, 214, 216, then at least one gate material, comprising for example at least one metallic material such as TiN and/or at least one semi-conductor material such as polysilicium, which may be highly doped, for example with Boron with a doping of around 10²⁰ atoms/cm³. The holes 210, 212, 214, 216 are filled by the gate material and this may then be followed by a CMP step (chemical mechanical polishing).

In one case for example where the Si_(1-y)Ge_(y) based semi-conductor zones 231, 232, 233, 234, are formed with a high composition in Germanium, for example such that y is close to 1, the depositing of the dielectric gate material may be preceded by a step to form a SiO₂ based interface layer with a thickness for example of between 0.3 and 3 nanometres. The interface layer may be made for example by the growth of a thin Si layer on the Si_(1-y)Ge_(y) based semi-conductor zones 231, 232, 233, 234 followed by oxidation of this thin Si layer.

In another variant of the embodiment of the method previously described in relation to the FIGS. 1A-1D, a masking layer 308 based on a dielectric material 309, for example such as SiO₂, and with a thickness for example of between 20 nanometres and 1000 nanometres (FIG. 3A) may be deposited on the Si_(1-x)Ge_(x) based semi-conductor layer 104. Subsequently, on the one hand the masking layer 308 is removed, so as to provide it with a form of a bevel. Following this removal, the upper face of the masking layer 308 creates an angle that is not nil with the main plane of the substrate 100 or forms an angled slope with respect to the main plane of the substrate 100 (the main plane of the substrate 100 is defined as a plane passing through the substrate 100 and parallel to the plane [O;{right arrow over (i)};{right arrow over (k)}] with an orthogonal reference frame [O;{right arrow over (i)};{right arrow over (j)};{right arrow over (k)}]). In one case, for example, where the dielectric material 309 is SiO₂, the removal may be made for example by soaking the substrate covered by the masking layer 308, in a HF+H₂O based solution, then by progressive removal of the solution (FIG. 3B).

Subsequently, the semi-conductor layer 104 is oxidised at a high temperature similar to that described in one or other of the embodiments of method previously described. The layer 308 in the form of a bevel acts as a mask for this oxidation. Following this oxidation, between the masking layer 308 and the layer 104, Si_(1-y)Ge_(y) based semi-conductor zones 331, 332, 333, (where y>x) that have different contents in Ge or different compositions in Germanium, are formed. A gradient of the concentration in Ge may be obtained in the semi-conductor thickness located between the insulating layer 101 and the masking layer, in a direction parallel to the main plane of the substrate 100. This gradient depends on the angle between the upper face of the masking layer 308 and the main plane of the substrate 100.

In another embodiment, a mask 408, comprising a plurality of blocks 410 a, 410 b, 412 a, 412 b, of which at least two blocks 410 a, 412 a, that have different compositions or are based on different materials, is made on the Si_(1-x)Ge_(x) based semi-conductor layer 104. The mask 408 may be formed for example by several blocks 410 a, 410 b, based on a first material 411 that may be dielectric such as for example SiO₂ and several other blocks 412 a, 412 b, based on a second material 413 that may be dielectric such as for example Si₃N₄. In this example, the masking blocks 410 a, 410 b, 412 a, 412 b, may have identical thicknesses. The blocks 410 a, 410 b, based on the first dielectric material 411 and blocks 412 a, 412 b, based on the second dielectric material 413 may be made using a method comprising for example several steps of depositing and photolithography (FIG. 4A).

Subsequently, an oxidation step of the semi-conductor layer 104, such as that previously described, is carried out through the masking blocks 410 a, 410 b, 412 a, 412 b. Due to their different compositions, the masking blocks 410 a, 410 b, based on the first dielectric material cause a delay in the oxidation of the different semi-conductor layers 102 and 104, and for example less than that caused by the masking blocks 412 a, 412 b, based on the second dielectric material. During the oxidation step, the condensation of the Germanium and the consumption of the Silicon are much quicker in the zones of the semi-conductor layer 104 located opposite the masking blocks 410 a, 410 b based on the first dielectric material 411 than in other zones of the semi-conductor layer 104 located opposite masking blocks 412 a, 412 b based on the second dielectric material 413.

Following the oxidation, we obtain under the masking blocks 410 a, 410 b, based on the first dielectric material 411, Si_(1-y)Ge_(y) based semi-conductor zones 431 a, 431 b, which have a first composition in Germanium, and a content in Germanium such that y=y₁ (where y₁>x), y₁ is for example around 0.8 to 1 under the masking blocks 412 a, 412 b based on the second material, other Si_(1-y)Ge_(y) based semi-conductor zones 433 a, 433 b, which have a second composition in Germanium different from said first composition. The semi-conductor zones 433 a, 433 b located under the masking blocks based on the second dielectric material, may have a content in Germanium that is lower than that of the semi-conductor zones 431 a, 431 b located under the masking blocks based on the first dielectric material and such that y=y₂ (where x<y₂<y₁), y₂ is for example around 0.1 to 0.3 (FIG. 4B).

In one specific embodiment, in the case of the mask 408 further comprising at least one opening 415 revealing the semi-conductor layer 104 and for example which has a transistor gate pattern, following the oxidation, we obtain opposite the opening 415 a Si_(1-y)Ge_(y) based semi-conductor zone 434, which has a third composition in Germanium, and a content in Germanium such that y=y₃ (where x<y₂<y₃<y₁).

In one variant, the oxidation mask may be formed of different materials, for example of blocks 410 a, 410 b based on a first dielectric material and blocks 412 a, 412 b based on a second material, that is metallic for example such as TiN.

Another variant of the embodiment of the method previously described in relation to FIGS. 2A-2D, will now be provided. For this variant, the masking layer 203 and the set of holes 210, 212, 214, 216, in this masking layer 203 are formed on the thin semi-conductor layer 102. Then a set of Si_(1-x)Ge_(x) based semi-conductor zones 504 a, 504 b, 504 c, 504 d, (where 0<x≦1, for example with x of around 0.1) which has an identical content in Germanium is made, for example by epitaxy at the bottom of the holes 210, 212, 214, 216, on the thin Si based semi-conductor layer 102 (FIG. 5A).

Subsequently (FIG. 5B) a first set of masking blocks 508 a, 508 b, 508 c is made, respectively filling one hole and covering a Si_(1-x)Ge_(x) based semi-conductor zone. The masking blocks 508 a, 508 b, 508 c, may be based on a dielectric material for example such as Si₃N₄. Among the Si_(1-x)Ge_(x) based semi-conductor zones 504 a, 504 b, 504 c, 504 d, at least one semi-conductor zone 504 c may be stripped bare or revealed and not be covered by a masking block.

A first oxidation step of the semi-conductor zones 504 a, 504 b, 504 c, 504 d, is carried out at high temperature, through a first mask formed by the perforated layer 203 and the first set of masking blocks 508 a, 508 b, 508 c. Depending on their respective thicknesses, the masking blocks 508 a, 508 b, 508 c, cause a delay that is more or less long to the oxidation of the semi-conductor layer 102 and the semi-conductor zones 504 a, 504 b, 504 c, 504 d. Once the first oxidation step is complete, a first semi-conductor zone 504 a located under a first masking block 508 a, a second semi-conductor zone 504 b located under a second masking block 508 b, and a third semi-conductor zone 504 c located under a third masking block 508 c, have a different content in Germanium from that of a fourth semi-conductor zone 504 d located in a hole 214 which does not have a masking block, and is revealed or stripped bare.

A second mask is then made. The second mask may be formed by a second set of masking blocks 518 a, 518 b, respectively filling one hole and covering respectively a Si_(1-x)Ge_(x) based semi-conductor zone. The number of masking blocks 518 a, 518 b, of the second mask, is different from that of the first mask. The number of masking blocks of the second set of blocks 518 a, 518 b may be less than that of the first mask. The second mask may be such that at least one semi-conductor zone 504 c that was located under a masking block 508 c of the first mask is now revealed. The second mask may also be such that at least one semi-conductor zone 504 d that was revealed during the first oxidation, remains revealed after the formation of the second mask. The second mask may be made by eliminating all of the blocks then by forming the new blocks or by elimination of one or several blocks of the first mask.

A second oxidation step of the zones 504 a, 504 b, 504 c, 504 d, is carried out at high temperature, through the second mask formed by the perforated layer 203 and the masking blocks 518 a, 518 b.

Once the second oxidation step is complete, the semi-conductor zones 504 a, 504 b, located respectively under the first masking block 508 a, under the second masking block 508 b, have a content in Germanium lower than that of the semi-conductor zones 504 c, 504 d revealed during the second oxidation. The content in Germanium of the semi-conductor zone 504 d that was revealed during the first oxidation and the second oxidation, is higher than that of the semi-conductor zone 504 c that was masked during the first oxidation and revealed during the second oxidation.

An embodiment of the method of the invention, enabling the fabrication of a microelectronic device equipped with several Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) which have different respective compositions in Germanium formed on a same substrate, will now be provided in relation to the FIGS. 6A to 6E. The starting material of the method may be a substrate of the semi-conductor on insulator type, for example SOI type (silicon on insulator) comprising a substrate layer 100 for example Si based, covered with a layer of dielectric material 101 for example a buried oxide layer, which itself is covered with a semi-conductor layer 602 for example Si based, and with a thickness of between for example 5 and 100 nanometres or for example around 20 nanometres.

Firstly the Si based zones of different thicknesses are fabricated on the substrate 100.

To this end, on the semi-conductor layer 602, a layer 608 may be formed that may be based on a dielectric material, such as for example SiO₂, and with a thickness for example of between 20 nanometres and 1000 nanometres. Subsequently, on the one hand the masking layer 608 is removed, so as to provide this layer 608 with a form of a bevel (FIG. 6A). After this removal, the upper face of the masking layer 608 forms an angle that is not nil with the main plane of the substrate 100 or forms an angled slope with respect to the main plane of the substrate 100 (wherein the main plane of the substrate 100 is defined as a plane passing through the substrate 100 and parallel to the plane [O;{right arrow over (i)};{right arrow over (k)}] with an orthogonal reference frame [O;{right arrow over (i)};{right arrow over (j)};{right arrow over (k)}]). In one case, for example, where the dielectric material of the layer 608 is SiO₂, the removal may be made for example by soaking the substrate covered by the masking layer 608, in a HF+H₂O based solution, then by progressive removal of the solution.

Subsequently, the semi-conductor layer 602 is oxidised, so as to provide a form of a bevel on the thin semi-conductor layer 102 (FIG. 6B). The bevel layer 602 thus comprises a plurality of Si based zones 602 a, 602 b, 602 c of different thicknesses resting on the substrate 100.

The masking layer 608 is then removed (FIG. 6C).

On the thin semi-conductor layer 102 another Si_(1-x)Ge_(x) based semi-conductor layer 614, where 0<x<y≦1, for example where x is around 0.1, with a thickness of between for example 50 and 100 nanometres or for example between 40 and 75 nanometres is formed for example by epitaxy (FIG. 6D). Then a localised condensation step of the Ge is carried out on the Si_(1-x)Ge_(x) based semi-conductor layer 614, for example using a heat treatment oxidation method such as that previously described. Following the oxidation or oxidations, Si_(1-y)Ge_(y) based semi-conductor zones 631, 632, 633 (where y>x) and which have different contents in Ge or different compositions in Germanium are formed. A gradient of the concentration in Ge may be obtained, in a direction parallel to the main plane of the substrate 100. The semi-conductor zones 631, 632, 633 may be covered with a thin SiO₂ based insulating layer formed during the oxidation. This insulating layer may then be removed (FIG. 6E).

In one variant (not shown) of the embodiment that has just been described, Si based zones of different thicknesses may be fabricated by forming a mask including a plurality of openings revealing the silicon layer 102, then forming, for example by several epitaxies, silicon blocks of different thicknesses at the bottom of the openings. The Si_(1-x)Ge_(x) based silicon blocks are then formed, where 0<x<y≦1, for example where x is around 0.1. Then, a heat treatment oxidation is carried out, such as that described above, on the Si_(1-x)Ge_(x) based blocks. Following the oxidation or oxidations, Si_(1-y)Ge_(y) based semi-conductor zones 631, 632, 633 (where y>x) with different contents in Ge or different compositions in Germanium are formed. 

1. Method for producing a microelectronic device including a plurality of Si_(1-y)Ge_(y) based semi-conductor zones (where 0<y≦1) which has different respective Germanium contents, comprising the steps of: a) the formation on a plurality of Si based semi-conductor zones and of different thicknesses rest on a substrate, of a Si_(1-x)Ge_(x) based semi-conductor layer (614) (where 0<x<1 and x<y), b) the oxidation of the Si_(1-x)Ge_(x) based semi-conductor layer.
 2. Method of claim 1, in which the semi-conductor zones of different thicknesses are zones of a Si based layer in the form of a bevel.
 3. Method of claim 2, wherein the bevel form of the Si based layer has been formed prior to step a) by: the formation of a mask in the form of a bevel on the Si based layer, oxidation of the Si layer through said mask in the form of a bevel, removal of the said mask in the form of a bevel.
 4. Method of claim 3, wherein the mask in the form of a bevel is based at least an insulating material such as SiO₂. 